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Dams, Water Quality and Tropical Reservoir Stratification

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Dams, Water Quality and Tropical Reservoir Stratification Biogeosciences, 16, 1657–1671, 2019 https://doi.org/10.5194/bg-16-1657-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Reviews and syntheses: Dams, water quality and tropical reservoir stratification Robert Scott Winton1,2, Elisa Calamita1,2, and Bernhard Wehrli1,2 1Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland 2Eawag, Swiss Federal Institute of Aquatic Science and Technology, 6047 Kastanienbaum, Switzerland Correspondence: Robert Scott Winton ([email protected]) Received: 13 December 2018 – Discussion started: 2 January 2019 Revised: 25 March 2019 – Accepted: 3 April 2019 – Published: 23 April 2019 Abstract. The impact of large dams is a popular topic in These changes and associated environmental impacts need environmental science, but the importance of altered water to be better understood by better baseline data and more so- quality as a driver of ecological impacts is often missing phisticated predictors of reservoir stratification behavior. Im- from such discussions. This is partly because information proved environmental impact assessments and dam designs on the relationship between dams and water quality is rel- have the potential to mitigate both existing and future poten- atively sparse and fragmentary, especially for low-latitude tial impacts. developing countries where dam building is now concen- trated. In this paper, we review and synthesize information on the effects of damming on water quality with a special focus on low latitudes. We find that two ultimate physical 1 Introduction processes drive most water quality changes: the trapping of sediments and nutrients, and thermal stratification in reser- As a global dam construction boom transforms the world’s voirs. Since stratification emerges as an important driver and low-latitude river systems (Zarfl et al., 2014) there is a se- there is ambiguity in the literature regarding the stratification rious concern about how competing demands for water, en- behavior of water bodies in the tropics, we synthesize data ergy and food resources will unfold. The challenge created and literature on the 54 largest low-latitude reservoirs to as- by dams is not merely that they can limit the availability of sess their mixing behavior using three classification schemes. water to downstream peoples and ecosystems, but also that Direct observations from literature as well as classifications the physical and chemical quality of any released water is of- based on climate and/or morphometry suggest that most, ten altered drastically (Friedl and Wüest, 2002; Kunz et al., if not all, low-latitude reservoirs will stratify on at least a 2011). Access to sufficient quality of water is a United Na- seasonal basis. This finding suggests that low-latitude dams tions Environment Programme sustainable development goal have the potential to discharge cooler, anoxic deep water, (UNEP, 2016), and yet the potential negative effects of dams which can degrade downstream ecosystems by altering ther- on water quality are rarely emphasized in overviews of im- mal regimes or causing hypoxic stress. Many of these reser- pacts of dams (Gibson et al., 2017). voirs are also capable of efficient trapping of sediments and Dams are often criticized by ecologists and biogeo- bed load, transforming or destroying downstream ecosys- chemists for fragmenting habitats (Anderson et al., 2018; tems, such as floodplains and deltas. Water quality impacts Winemiller et al., 2016), disrupting floodplain hydrologic cy- imposed by stratification and sediment trapping can be mit- cles (Kingsford and Thomas, 2004; Mumba and Thompson, igated through a variety of approaches, but implementation 2005; Power et al., 1996) and for emitting large amounts of often meets physical or financial constraints. The impending methane (Delsontro et al., 2011). Such impacts act against construction of thousands of planned low-latitude dams will the promise of carbon-neutral hydropower (Deemer et al., alter water quality throughout tropical and subtropical rivers. 2016). In contrast, scientists have committed less investiga- tive effort to documenting the potential impacts of dams on Published by Copernicus Publications on behalf of the European Geosciences Union. 1658 R. S. Winton et al.: Dams, water quality and tropical reservoir stratification water quality. In cases where investigators have synthesized knowledge of water quality impacts (Friedl and Wüest, 2002; Nilsson and Renofalt, 2008; Petts, 1986), the conclusions are inevitably biased towards mid/high latitudes where the bulk of case studies and mitigation efforts have occurred. While there is much to be learned from more thor- oughly studied high-latitude rivers, given the fundamental role played by climate in river and lake functioning, it is im- portant to consider how the low-latitude reservoirs may be- have differently. For example, the process of reservoir strat- ification, which plays a crucial role in driving downstream water quality impacts, is governed to a large degree by lo- cal climate. Additionally, tropical aquatic systems are more prone to suffer from oxygen depletion because of lower oxy- gen solubility and faster organic matter decomposition at higher temperatures (Lewis, 2000). Latitude also plays an important role in considering ecological or physiological re- sponses to altered water quality. Studies focused on cold- water fish species may have little applicability to warmer rivers in the subtropics and tropics. Low-latitude river systems are experiencing a high rate of new impacts from very large dam projects. A review of the construction history of very large reservoirs of at least 10 km3 below ±35◦ latitude reveals that few projects were launched between 1987 and 2000, but in the recent decade (2001–2011) low-latitude mega-reservoirs have appeared at a rate of one new project per year (Fig. 1). Given that ongo- ing and proposed major dam projects are concentrated at low Figure 1. Construction history of the world’s 54 largest reservoirs latitudes (Zarfl et al., 2014), a specific review of water quality located below ±35◦ latitude. Project year of completion data are impacts of dams and the extent to which they are understood from the International Commission on Large Dams (https://www. and manageable in tropical biomes is needed. icold-cigb.net/, last access: 1 November 2018). Project start data are Large dams exert impacts across many dimension; but approximate (±1 year) and based on either gray literature source in this review, we largely ignore the important, but well- or for some more recent dams, i.e., visual inspection of Google covered, impacts of altered hydrologic regimes. Instead, we Earth satellite imagery. Grand Ethiopian Renaissance abbreviated as GER. Volume in map legend is in cubic kilometers. focus on water quality, while acknowledging that flow and water quality issues are often inextricable. We also disregard the important issue of habitat fragmentation and the many acute impacts on ecosystems and local human populations arising from dam construction activities (i.e., displacement on physical and chemical processes within the reservoirs that and habitat loss due to inundation). These important topics may affect downstream water quality. have been recently reviewed elsewhere (e.g., Winemiller et Finally, we review off-the-shelf efforts to manage or mit- al., 2016; Anderson et al., 2018). igate undesired chemical and ecological effects of dams re- In order to understand the severity and ubiquity of wa- lated to water quality. The management of dam operations ter quality impacts associated with dams, it is necessary to to minimize downstream ecological impacts follows the con- understand the process of lake stratification, which occurs cept of environmental flows (eflows). The primary goal of because density gradients within lake water formed by so- eflows is to mimic natural hydrologic cycles for downstream lar heating of the water surface prevent efficient mixing. By ecosystems, which are otherwise impaired by conventional isolating deep reservoir water from surface oxygen, stratifi- dam-altered hydrology of diminished flood peaks and higher cation facilitates the development of low-oxygen conditions minimum flows. Although restoring hydrology is vitally im- and a suite of chemical changes that can be passed down- portant to ecological functioning, it does not necessarily stream. To address the outstanding question of whether low- solve water quality impacts, which often require different latitude reservoirs are likely to stratify and experience asso- types of management actions. Rather than duplicate the re- ciated chemical water quality changes, we devote a section cent eflow reconceptualizations (e.g., Tharme, 2003; Richter, of this study to predicting reservoir stratification. This analy- 2009; Olden and Naiman, 2010; O’Keeffe, 2018), we focus sis includes the largest low-latitude reservoirs and is focused our review on dam management efforts that specifically tar- Biogeosciences, 16, 1657–1671, 2019 www.biogeosciences.net/16/1657/2019/ R. S. Winton et al.: Dams, water quality and tropical reservoir stratification 1659 are not well understood. Collecting the reservoir depth pro- files necessary to generate this key information may be more difficult than simply analyzing water chemistry below dams. Dam tail waters with low oxygen or reduced compounds, such as hydrogen sulfide or dissolved methane, are likely to stem from discharged deep water of a stratified reservoir.
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